The working relationship between the US and Vietnamese sides is “at such a level now that really allows us to expand and move forward on our operations here,” said Lieutenant Colonel Patrick Keane, commander of JPAC’s Hanoi detachment.

The US and Vietnam began co-operating on investigations into missing American servicemen in 1985, helping to pave the way for a normalisation of diplomatic relations 10 years later.

On Thursday, investigators marked the 10th anniversary of a helicopter crash which killed seven American military men and nine Vietnamese on a joint mission to search for US MIAs.

Senior officials from both nations, along with relatives of the dead, joined the ceremony at a commemorative monument near the crash site in the north-central province of Quang Binh.

Since the end of US combat involvement in 1973, the remains 668 Americans listed as missing during the war have been repatriated from Vietnam and identified, but 1,303 are still unaccounted for, the US says.

Hanoi says about 300,000 North Vietnamese soldiers are also still listed as missing from the war.